Circuit control means



C. L. STROUP CIRCUIT CONTROL MEANS 2 SHEETS-SHEET l April 15, 1952 Filed NOV. 9, 1946 April 15, 1952 c. L. sTRoUP 2,593,214

CIRCUIT CONTROL MEANS Filed Nov. 9, 194e 2 SHEETS- SHEET 2 Patented Apr. 15, 1952 UNITED STATES PATENT OFFICE CIRCUIT CONTROL MEANS Charles L. Strom), Oak Park, Ill.

Application November 9, 1946, Serial No. 708,860

Claims.

My invention relates, generally, to circuit lnterrupting devices, and it has .particular relation to circuit .breakers for opening the `circuit to the winding of a load device, such as the primary winding of a transformer. This invention constitutesan improvement over the inventions disclosed in my co-pending -applications, Serial No. 568,279 .filed December 15, `1944, land .Serial No. 642,215 .led January 19, 1946, :now Patents 2,457,939 and 2,543,712, respectively.

In alternating current distribution systems, it is cutomary to supply several distribution transformers through a single circuit breaker which constitutes Ia main circuit breaker with respect to the `circuit breakers or interrupting devices which are individual to the transformers. Each transformer is connected individually to supply several customers. The Aprimary winding of each transformer' may be energized atarelatively high voltage, such as '2,300 to v4,000 volts or higher, while the secondary winding is energized at a relatively low voltage, such -as 110 to'220 volts. When the main circuit breaker is opened for any reason, such as in response rto a fault on the system, all ofthe ltransformers supplied thereby are deenergized This results in cutting olf the current iiow toall of the customers supplied by the several transformers.

Obviously, the main circuit breaker should be opened only in case of emergency. It is customary to provide for reclosing the main circuit breaker a number of times after a fault occurs `in an attempt to maintain the :service in the event that the fault is of a momentary lor transient character. Provision is made for .locking out the main circuit breaker only after repeated attempts have been made to restore the service. Such `repeat operations of the main circuit breaker may vbe automatic, in which case the circuit breaker goes through a'predetermined time-cycle of reclosure operations. The operation of the circuit breaker may be manually effected las vwill be .readily understood.

'The fault often is of a temporary or transient character. VA bird or a squirrel may temporarily short circuit ithe high voltage terminal ofthe distribution transformer to ground. This may be suflicient to cause suflicient current flow to effect operation of the main circuit breaker. It opens the circuit and, in .the meantime, if the fault has been cleared by the bird or squirrel Adropping away, the circuit breaker may bereclosed and service may .be restored. It kmay take more `than one reclosure to destroy the yforeign object causing the flow of fault `current so that V2 repeated reclosures are required for accomplishing the same.

In the event that the fault should persist at a particular transformer after one or two reclosures of the main circuit breaker, Yit is desirable to disconnect this transformer so that lservice may be maintained on Vthe other transformers. This limits the inconvenience to the customers served by a single transformer. Such a Yfault may be in the form of a cracked insulator Vbushing on the transformer, or a punctured transformer coil, or short circuted transformer coil turns. To take care of isolating the individual transformer on the occurrence of such a fault, a clrcuit interrupter individual thereto is provided.

It is desirable to open the circuit to each transformer after the same has operated under limited overload conditions for an appreciable length of time. When the load device, such as the transformer is cold, it can be overloaded safely for -a short interval. .For example, it is possible to operate a transformer at overload for an hour or two Without damaging it, provided that -its temperature does not exceed predetermined allowable limits.

Small overload conditions Amay be simulated by the short circuiting of one or ya Vfew .turns rof the transformer windings. This will cause the transformer to overheat and may eventually result in the destruction of the transformer Vinsulation. Such a slight overload is accompanied by a slight increase in the current flow 'through the high voltage or primary winding of the ltransformer.

From the standpoint of economics it is impractical to provide a circuit interrupter for each transformer which has the circuit interrupting capacity ofthe main circuit interrupter that -is common to a number-of transformerinstallations. Accordingly, the interrupting capacity of the circuit interrupting device for each transformer is substantially less than that of the main circuit interrupter. On occasion, the :flow of fault current at a transformermay be of such a magnitude that it cannot be interrupted safely by the circuit interrupter individual thereto, but it can be interrupted by the main circuit breaker.

However, after the fault has been cleared by the main circuit breaker, it is desirable that the circuit interrupter individual to the transformer open the circuit thereto so that, when the main circuit breaker is reclosed, either automatically or manually, the fault will not remain on the system. Such operation is in the `interestof providing maximum continuity of service to the customers. which are not supplied by the particular transformer Where the fault is present.

When the flow of current to the primary winding of the transformer is utilized to determine when the circuit interrupter individual thereto should be opened, various difficulties are encountered. The current flow to the transformer under slight overload conditions may be of the order of only a few amperes. The normal load current may be of the order of one ampere. While it is desirable to provide means which will be responsive to the flow of 11/2 to 2 amperes for tripping the circuit interrupter individual to the transformer, provision must also be made for tal:- ing care of the flow of a wide range of fault currents which may be of the magnitude of several hundred or even thousands of amperes. The same current responsive device which will operate satisfactorily in response to the ow of a few amperes may be injured or totally destroyed if it is subjected to the flowV of several hundred amperes. Y

Accordingly, among the objects of my invention are:y lTo effect the operation of a circuit interrupter in'response to the now of small or large fault currents without damage to the current responsive means; to limit the current flow tothe current responsive nteans-to a predetermined -maximum regardless of the magnitude of thel'low of fault'current; to energize directly a hydraulic relay as a result of the flow of current, which is 'a function of the fault current, through a portion of the hydraulicrelay; to operate the hydraulic relay in accordance with the timecurrent characteristics of the load device so as to open the circuit interrupter only after the load device reaches a dangerous operating condition; to prevent the circuit interrupter from opening the circuit on the flow of fault current in excess of a predetermined magnitude andv thereafter to trip the circuit interrupter on the cessation of such excess Ycurrent flow; to permit manually tripping of the circuit interr-upter without operating the automatic trip mechanism; and to provide a spark gap across the circuit interrupter, .the terminals of which are formed by parts of the trip mechanism'.

'Other objects'of my invention will, in part, appearhereinafter. My invention isv disclosed in the embodiment therecf'shown in the accompanying drawings and it comprisesthe features oflco'nstruction, combination of elements and arrangement of partsl which will be exemplified in the construction hereinafter set forth and the scope of the application of-which will be indicated inthe appended claims.

For a more complete understanding of the nature and scope of my invention, referencemay be had to the following detailed description, taken together vwith the accompanying' drawings, in

which: Y Y l I Figure' 1 is a View, in side elevation, of a conventional transformer of the single insulator lbushing type on which is mounted a circuit in- Y trates one form of automatic holding means that may be employed in practicing my invention;

Figure 4 is a detail sectional view taken along -thevline Ilof- Figure 3; and

Figure 5 illustrates, diagrammatically, the circuit connections which may he employed in practicing my invention.

Referring now particularly to Figure 1 of the drawings, it will be observed that the reference character 9 designates, generally, the transformer which may be of the conventional cil insulated single phase type having a metal case i6 that is grounded at il. The transformer 9 may be of the single insulator type and may have an insulator bushing l2 extending upwardly from the cover thereof, as illustrated. The insulator bushing i2 supports a high voltage terminal rod i3 from which a bar le of copper or the like extends.

The bar i4 is arranged to be engaged by the down turned end of aterminal bar i5 which constitutes one terminal of a circuit interrupter indicated, generally, at I6 that is constructed in accordance with the present invention. The other terminal ofthe circuit interrupter I6 is indicated at I1, and it is arranged to be connected to a high voltage conductor, as will be readily understood. i

In Figure 2 of the drawings the details of construction of the circuit interrupter it are shown more clearly. It will be observed that it includes an oil pot, designated generally at 2i), whichY is made up of an inner shell 2i of conducting material, such as' copper, and which is surrounded by suitable heat insulating material 22. A sheath 23 of metal 'surrounds the covering 22 of insulating material. The shell 2| is sealed at its lower end byA a metal bottom- 24 which carries immediately thereabove a disc 25.01 insulation'. Mounted on the disc 25 is a generally U-shaped contact 2B which forms one of a pair of separable contacts that are arranged to, be opened under certain conditions as will be described in detail hereinafter. The other contact 29 is in theform of a rod of suitable conducting material, such as copper or brass, and constitutes an extension of the terminal l1, previously referred to. The contact 29 is arranged to separate from contact 26 under oi1,the level of which is indicated at 30. The oil may be any suitable well-known insulating liquid that is employed for circuit interrupting purposes. At its upper end the shell 2l' is closed by a discv 3l of insulating material, into which is threaded `a tube or sleeve 32 of insulating material, such as fiber, which, when subjected to the heat of `the arc drawn between the contacts 2S and 29, evolves an arc extinguishing medium that assists in extinguishingA the' arc. LSurrounding the tube 32 is an insulator 33 which is vformed of porcelain or other similar weather resisting material. c

AV metal sleeve 34 depends from the upper end of the tube 32 and is clamped in position thereon by a clamp nut 35. At the lower end of the sleeve 34 is secured a thimble 36 against which a coil compression spring 31 reacts for biasing the contact 29 upwardly. The upper end of the spring '31 bears' against the `underside of a nut 38 that is'threaded on the upper end of the contact 29. AUnless restrained, as vdescribed hereinafter, the spring 3'i' would move the contact 29 upwardly to separate the same from stationary. contact 2t. Thexupward movement is limited by a stop ring 39,- secured to the contact29, when it engages the'underside of the thimble 36. j f Above the nut 33 there is Vprovided a metal cap 40 which moves with the contact 29 and whichextends downwardly over the upper end fof vthe insulator 33. The cap 4! serves to protect the upper -end 'of the insulator 33 and parts associated therewith from the weather. A clamp nut 4| serves to hold the cap 40 in place. Another nut 42 is provided for clamping Athe line conductor between it and the nut 4| so as to make the necessary electrical connection to the terminal and to the contact 29 which forms a part thereof.

A strap 46 of flexible resilient conducting material is clamped between the nuts 38 and 4|, as shown, and forms one part of a multi-part connector which is employed in the latch mechanism, which will be vdescribed presently. The strap 45 is secured, as by riveting, to one end of a bar A4'| of insulation. At the other enol of `the bar r4`| is a metal lfitting 48 which has -a foot portion -49 that Yis arranged'to be inserted between the bifurcated ends 50 of a latch 'lever l5|. In my copending application, Serial No. 642,215, I have shown in Vmore detail a preferred construction for the bifurc'ated ends 5|) of the latch lever 5|. The latch lever may be pivoted at 52 in a metallic housing 53 which is carried by the terminal bar I5. Integrally Aformed with the latch lever 5| is a nose portion 54 that is arranged to 'have latching engagement with one arm 55 of a bell-crank which may be pivoted at 55' on ythe housing 53. The other arm 56 of the bell-crank is 'arranged to be engaged by a'plunger 57, which is operated on expansion of a flexible bellows '58 that is located at the lower end of the housing 53.

rThe bellows 58 forms a part of a hydraulic relay which is indicated'generally, at 59 in Figure 5 o'f the drawings. Within the bellows 58 there is provided a tubular compression member llwhich serves to limit the amount that the bellows 58 can contract. Since it is desired thatth'e bellows 58 expand to perform the tripping function only when the relay 59 is subjected to a predetermined temperature, the compression member 60 is provided to preload the same and prevent unnecessary operation of the bellows 58 below this temperature. The compression member 60 has apertures 6| therein to permit circulation of fluid therethrough.

It will be observed that the bellows 58 is connected by a capillary tube 52 to a bulb 63 which may be in the form of a cylindrical gas-tight container. It is preferably formed of conducting material, such as copper. The liquid within the bulb 63, which substantially fills it andfills the -tube 62 and bellows 58. is indicated at 64. A gas 4occurs on the line to which the terminal il is connected. For this purpose the tting 48 is provided with an integrally formed outwardly inclined extension S9, and likewise, the strap 45 is provided with a similarlypositioned extension to form a spark gap 1| therebetween. The dimensions of the spark gap 7| are such that a lightning surge or other transient high voltage wave will discharge thereacross.

Another function of the upward and outwardly extending portion 69 of the fitting 43 is to yprovide for receiving the prong of a switch stick. This permits the foot portion 49 to be swung manually downwardly into engagement with the bifurcated ends 53 of the latch lever 5|. Because of the flexible resilient strap 46 this also permits the foot portion 48 to be'unlatched from the latch lever 5| without operating the latch mechanism so as to permit movement of the contact 29 out of engagement with the contact 26. Thus, it is unnecessary to operate manually the bell-crank formed by the arms 55 and 5B for unl'atching the foot portion 49 and releasing the .contact 29 for movement to the open circuit position.

The construction of the latch mechanism is such that it is tripped or unlatched when the bulb 63 is heated to a predetermined temperature. For example, the design may be such that the flexible bellows 58 does not further expand until the bulb 63 is subjected to a temperature of, .for example, F.

While it is desirable to provide for heating the bulb 63 in accordance with the flow of current through the circuit interrupter I6, itis necessary. as indicated hereinbefore, to take care of `a wide range currents, i. e., ranging from a few amperes to several hundred or even thousands of amperes, and still avoid injury to the current responsive mechanism. Accordingly, instead of directly heating the bulb 63 'by the line current, as disclosed in my co-pending application, Serial No. 642,215, I have provided a control or current transformer, which is indicated, generally, at 13 having a primary winding 'i4 that is connected by terminal conductors 75 and 75 in series with the contacts 25 and 2e. Specifically, the terminal conductor l5 is connected to the stationary contact 26 while the conductor 16 is connected to the shell 2| that, in turn, is connected to the terminal bar |5.

The transformer 73 also includes a saturabie magnetic core on which the primary winding i4 is wound. The core ll has a single turn secondary winding 'I8 surrounding the same and connected across the bulb 53. The size of the magnetic core 'il is such that, regardless of the amount of current flow through the primary winding 74, the current flowI through the single turn secondary winding i8 will not exceed a predetermined value. For example, the design may be such that the core 'il will saturate so that the current iiow induced into the single turn secondary Winding 18 will not exceed 500 amperes.

During normal operating conditions the current flow through the primary winding 'le does not induce sufiicient current into the single turn secondary winding 'i8 to produce a heating effect sufficient to raise the temperature of the bulb 63 above the critical temperature. However', when the current flow through the primary winding T4 increases to fault current proportions, enough current is induced into the secondary winding 'I8 to produce suihcient resistance losses in the `bulb 63 to heat the gas in the gas space 65 and cause the liquid 54 to be driven into the bellows 58. As a result, the bellows 58 expands, the bell crank lever comprising the arms 55 and 56 is rocked to the tripped position and latch lever 5| is released. The foot 49 then swings out of engagement with the bifurcated ends 55 of the latch lever 5| and, under the influence of the coil compression spring 3l, the contact 25 is moved upwardly to interrupt the circuit.

As indicated hereinbefore, the contacts 26 and 29 are separated under oil. This serves to extinguish the aro which is incident to the operation of the circuit interrupter I8.

It will be observed that the bulb '63 is located in the oil which is used for assisting in circuit interruption. When the bulb 63 is in intimate contact with the oil, its temperature is correspondingly aected. Accordingly, it is possible to damp or retard the action of the hydraulic relay 59 in accordance with the amount of oil that is adjacent the bulb 93. This makes it possible to duplicate in the circuit interrupter IB, the time-current characteristics of the transformer 9. Initially, when the transformer 9 is cold, it can be overloaded to a limited extent for a limited period. The design of the circuit interrupter IB may be such that, as the overload current flows therethrough to the transformer,

the time-temperature changes therein will duplicate those in the transformer 9. But, after the transformer 9 has been subjected to a slight overload for a predetermined time, the lbulb 93 w'illiinally be heated to its critical temperature atthe time after which it would be unsafe tc continue the overload on the transformer. Thereupon thev hydraulic relay 99 operates by expansion of the bellows 98 to trip the circuit interrupter I6, as described above.

When the construction disclosed herein is employed, it is possible to so design it that, when the normal full-load current of the transformer is' of the order of 0.7 amperes, and the transformer is capable ofwithstanding a 100% overload for one hour, the hydraulic relay 59 will function at the end of this period in response to the flow vof 1.4 amperes through the primary Winding 14 to tripthe circuit interrupter i9 and open the circuit.

While the circuit interrupter can be designed so as to be sensitive to this relatively small increase in current flow, the construction is such that it is also capable of withstanding the iiow of relatively high fault current. The primary winding 1d can be constructed of the required number of turns of a relatively large cross-see tion conductor so as to be capable of carrying, for short periods of time, relatively great fault currents. However, because of the saturable magnetic core 11, the amount of current induced into the single turn secondary winding 19 is limited, as described hereinbefore. Under such .operatingconditions on the occurrence of a relatively severe fault, the bulb 63 is quickly heated to above lits critical temperature and the hydraulic relay 59 immediately performs the tripping function. Forexample, under short circuit operating conditions, it can be designed so that it will trip within one second after the fault lcurrentfstarts to flow. y `In'ligure 5 of the drawings the circuit details of the invention are illustrated diagrammatically. It will be noted that the main transformer windings are indicated, generally, at 89 and that they` include a lprimary winding t! which may be connected by a conductor 82 in series circuit relation with the primary winding 14 of the vcontrol or current transformer 13. A conductor 83 may be provided for connecting the other terminal of the primary winding 2li to ground H. The secondary or low voltage winding of the transformer 99 is indicated at 84. It will be understood that this winding is connected to energize the various circuits-of the customers or to other loads supplied by the transformer 9. The `high voltage line conductor is indicated at 85.

With a View to providing a wider range in the design characteristics of the circuit interrupter I6, a second secondary winding S9 may be wound on the core 11 of the control transformerl for venergizing a heater 81 whichis immersed in the c luor insulating liquid surrounding the bulbv. x

Because of the use of the saturable magnetic core 71 there is no danger that the secondary Winding 89 or heater 81 will be injured on the flow of heavy fault current through the primary winding 14. By this means it is possible to provide a. wider range in the time-current characteristics `of the circuit interrupter I6 than is otherwise the case when reliance is placed solely on the `flow of current Vthrough the single turn secondary Winding 18 and bulb 63 for heating the latter to and above the critical operating temperature. Depending upon the design requirements, the heater 81 can be placed in close proximity to the bulb 93 Vor spaced away therefrom within the oil surrounding the same. In some instances, the single turn secondary winding 18 may be omitted altogether and reliance placed solely upon the heater 81 for heating the bulb 63.

As indicated hereinbefore, the flow of fault current through the circuit interrupter I9 may be such that the same would be injured or destroyed in the event that it should attempt to interrupt the same. For example, a bird or a squirrel may cause a dead short circuit between the bar i4 andthe tank 9 so that a fault current of the order of several hundred or even thousands of amperes may flow. Under such conditions, it is desirable that the circuit be interrupted by the main circuit breaker, which is provided with ample interrupting capacity, rather than by the circuit interrupter IB. For this purpose, it is necessaryl to provide means for preventing the separation of thecontact 29 from the'contact 26 even though the hydraulic-relay. 59 functions :to trip the latch mechanism.

For-this purpose a third secondary winding 99 may be-Wound on the core 11 of the transformer 13. It may be arranged to energize the operating Winding 99 of a holding mechanism, the details of construction of which are illustrated more clearly in Figures 3 and 4 of the drawings.v The operating winding 99 is arranged to attract an armature 9i which is restrained by a spring 92. A yoke 93 is moved by the armature 9| and, when suicient current flows through the operating winding 99, it is moved toward the contact-29 to they position shown inv Figure 4, where it is .located in the path of shoulder 99 that may be formed integrally with the contact 29. Y

Now, whenl fault current above a predetermined magnitude, for example, about 50 amperes,

flows through the primary winding 14, suiicient current is induced in the secondary winding 89 to energize the operating winding 90 andl to overcome the force of the Ispring 92 to move the yoke 93 into the path of the shoulder 94. Shortly thereafter, the bulb 63 is heated, as described hereinbefore, above its critical temperature and Ythe hydraulic relay 99 functions to trip the latch mechanism. However, although the foot 49 is released from the latch lever 5|, the contact 29 cannot move upwardly since it is restrained by the yoke 93 engaging the shoulder 99. As soon as the main circuit breaker operates to interrupt the circuit, the current iiow through the primary winding 19 ceases. The current now. through the secondary winding 89, likewise, ceasesv and operating Winding 99 is deenergized. The spring 92 then operates the yoke 93 out of the path of the shoulder 94 and permits spring 31 to move the contact 29 out of engagement with the contact 29. Since the circuit is deenergized, no arc is drawn between the contacts 26 and 29.

The main circuit breaker noW can reclose to energize whatever other transformersmay ybe 9 connected thereto since thefault individual to the particular transformer where-the circuit interrupter I6 has operated -Will isolate the same from the rest of the system. v y V Since certain further changescan be made in the foregoing construction and diiierent embodiments of the invention can befmade without departing from the spirit and scope thereof, it is intended that all matter shown in the accompanying drawings and described hereinbefore shall be interpreted as illustrative and not in a limiting sense. A

I claim as my invention: Y

1. Means for interrupting thecircuitinterconnecting a load device anda conductor energized with alternating current comprising, in combination, separable contacts adapted to be connected in said circuit-*spring means for effecting separation of said contacts, latch means adapted normally to maintain said contacts closed against the separating force of said spring means; a transformer having a primary winding connected in series with said contacts in said circuit, two secondary windings, and a saturable core whereby the current now in said secondary windings is limited; hydraulic relay means connected in circuit with one of said secondary windings and operatively connected to said latch means, and a heater connected in circuit with the other of said secondary windings adapted to heat a body of liquid surrounding said relay means for energizing said hydraulic relay means, said hydraulic relay means being responsive directly to the current flow in said one secondary winding and indirectly to the current flow in said other secondary winding for tripping said latch means.

2. Means for interrupting the circuit interconnecting a load device and a conductor energized with alternating current comprising, in combination, separable contacts adapted to be connected in said circuit, spring means for efiecting separation of said contacts, latch means adapted normally to maintain said contacts closed against the separating force of said spring means; a transformer having a primary winding connected in series in said circuit, first, second, and third secondary windings on said transformer, and a saturable core whereby the current now in said secondary windings is limited; hydraulic relay means connected in circuit with said rst secondary winding and operatively connected to said latch means, a heater connected in circuit with said second secondary winding and arranged to energize said hydraulic relay means through a body of liquid surrounding both said heater and relay means, said hydraulic relay means being responsive directly to the current flow in said first secondary winding and indirectly to the current flow in said second secondary winding for tripping said latch means on low of predetermined current in said circuit for a predetermined time, and holding means connected in circuit with said third secondary winding and operatively responsive to said current ow in excess of a predetermined minimum for preventing opening of said contacts as long as said excess current ows.

3. Circuit interrupting means according to claim 2 wherein the latch means for effecting separation of said contacts includes a tripping mechanism conductively connected to one of the contacts, and wherein there is a multi-part connecting member secured at one end to the other of said contacts and restrained at the other end by said tripping mechanism, and one of the parts of said connecting member being formed of resilient exible material whereby said connecting member is manually releasable from said tripping mechanism without operating the same to permit separation of said contacts.

el.y Circuit interrupting means according to claim 2 wherein the'latch means for effecting separation or" said contacts includes a tripping mechanism conductively connected to one of the contacts, and wherein there is a multi-part connecting member secured lat one end to the other of said contacts and restrained at the other end by'said tripping mechanism, one of the parts of said connecting member being formed or' resilient flexible material whereby said connecting member is manually releasable from said tripping mechanism without operating the same to permitseparation of said contacts, and the resilient part of the connecting member being a conductor connected to one of said contacts which is movable and having an extension which forms one terminal of a spark gap, the other spark gap terminal being formed by a conductor secured to another part of said connecting member which is oi insulating material and arranged to be held by the tripping mechanism.

5. A thermally responsive circuit interrupting attachment for opening the circuit to the windings of a load device to be protected and which is connected in circuit with a source of electrical potential, comprising, a hollow pot member adapted to hold a body of insulating liquid and having an inner liner of conductor material, a lower connector terminal insulated from said pot and in electrical contact with said liner for connecting said attachment in circuit with said load device, a stationary contact in the bottom interior of said pot and insulated from said liner, a second movable contact above said stationary contact comprising a rod member of conductor material adapted to move vertically toward and away from said stationary contact, an insulating cap `iournalling the upper portion of said rod and covering over the top of said pot, thimble means within said insulator and surrounding said contact rod, compressive spring means surrounding the upper end of said rod and having its lower end bearing against said thimble means, the upper end of said spring means being secured to the upper end of said rod; a second connective terminal means comprising the upper end of said rod for connecting said apparatus in circuit with an electrical supply line conductor, a multi-part connector secured at one end to said second terminal and contact rod comprising a flexible strap of conductor material and an additional strap of non-conductor material; a tripping latch mechanism outside of said pot and adjacent one side thereof, the lower end of said non-conductive part of said connector being adapted for latching engagement with said tripping mechanism whereby said spring means resiliently may be compressed to hold said contacts together; a control transformer within said pot having secondary winding and a primary winding which is connected in series with said two contacts through said liner, and a thermally responsive relay means submerged in said fluid within said pot, said relay comprising a conductive bulb inductively connected across said secondary winding, partially iilled with liquid and having a body of gas at the upper end therewithin, and a tubular member connecting the bottom of said bulb with an expansible bellows 11 mounted externally of said pot and operatively associated with said tripping mechanism whereby upon the excitation of said secondary winding in response to a predetermined current flow in said primary Winding of said control transformer, said gas will rapidly expand forcing said liquid within said bulb to ow into said bellows` via said tubular member thereby to expand said bellows, trip said tripping mechanism to release said connector and cause said two contacts to separate thereby to interrupt the circuit to said load device windings.

CHARLES L. STROUP.

REFERENCES CITED The following references are of record in the ille of this patent:

- UNITED STATES PATENTS Number Y Name Date 488,123 Keating Dec. 13, 1892 656,680 Thomson Aug. 28, 1900 Number Number 20 309,895

Name Date Baruch Oct. 15, 1918 McGahan Jan. 30, 1923 Wensley July 28, 1925 Wittingham Dee. 28, 1926 Wood Dec. 25, 1928 Laubet Sept. 2, 1930 Persons May 3, 1932 Schweitzer Sept. 20, 1932 Cooney Sept. 8, 1936 Van Antwerp May 17, 1938 Schwager Feb. 23, 1943 Schaefer Feb. 20, 1945 Van Sickle Mar. 20, 1945 Jackson July 2, 1946 Stroup Jan. 4, 1949 FOREIGN PATENTS v Country Date Germany Nov. 17, 1917 

